Polymeric compositions are extensively used as insulation materials for high voltage wire and cable. Olefin homo- and copolymers are the insulation materials of choice for high voltages (5000+volts) based on desirable electrical and physical properties such as dielectric strength; toughness, such as resistance to cutting and abrasion; ageing characteristics; resilience; and minimal cracking on mechanical stress. Polyethylene, polypropylene and blends thereof polymerized in different densities as well as copolymers thereof with each other and other insulating polymers are usually used.
It has been noted that such polyolefin-based materials when used as insulation materials in high voltage distribution wires and cable are prone to electrical failure including breakdown under voltage stress in wet environments. The latter breakdown may result from a degradation phenomenon identified as "water trees". Under high voltage wet-stress, microscopic channels, i.e. dendritic voids, appear in the insulation. These have a tree-like appearance - hence, the name. Such failure is most disadvantageous. This problem is now aggravated in that many high voltage cables are buried for greater reliability (decrease of damage from high winds, ice storms, etc.) and for aesthetic reasons. Over extended periods of time, short circuits have occurred in such buried cables resulting in loss of service. These cables have to be removed by excavation and replaced, a time consuming and costly operation.
Many classes of chemical compound additives have been disclosed in the prior art as effective voltage stabilizers, i.e. suppressants for electrical failure, water-treeing and/or electrical-treeing (microscopic dentrites caused by corona arcing). These prior art teachings include voltage stabilizers based on silicon derivatives, furfuryloxy phosphites and high-molecular weight polyethylene oxide.
In the prior art, Kowasaki, U.S. Pat. No. 4,305,849, teaches the use of polyethylene glycols having molecular weights of from about 1,000 to 20,000 as voltage stabilizers.
Ashcraft et al in U.S. Pat. Nos. 4,144,202 and 4,263,158 teach the use of organosilane compounds containing azomethine groups as voltage stabilizers.
Turbett et al in U.S. Pat. No. 4,376,180 disclose the use of 3-(N-phenylaminopropyl-tridodecyloxysilane) as a voltage stabilizer.
Turbett in U.S. Pat. No. 4,440,671 discloses the use of a blend of hydrocarbon-substituted diphenyl amine and a high molecular weight polyethylene glycol for this purpose.
Braus et al in U.S. Pat. No. 4,514,535 disclose the use of tritetrahydrofurfuryloxy phosphite as a voltage stabilizer.
Beasley et al in U.S. Pat. No. 4,374,224 disclose the use of an organic carboxylic ester having at least one aromatic ring and at least three carboxylic ester groups as a voltage stabilizer.
U.S. Pat. No. 3,553,348 describes the use of filler minerals such as magnesium silicate, pretreated with alkyl and vinyl alkoxysilanes, as voltage stabilizers.
It will be noted from most of these prior art teachings that previous experimenters have relied upon a test based on microscopic examination of the voltage stressed polymer. The number of "trees" and their length were utilized to judge the resistance deterioration of the insulation. These tests were based on the assumption that "tree" length and the number of trees could be used to approximate the relative useful life of the insulation. These tests, while rapid, are subject to many variables not quite analagous to the actual service conditions.
In contrast, the polymers utilized in this invention and representative compounds of the above mentioned prior art have been tested and compared herein by the use of actual wires coated with the test material compositions. These wires are immersed in a water bath and subjected to high voltage until electrical failure. This test methodology is more closely analagous to the conditions leading to electrical failure of buried distribution cables.